Tight oil reservoir in mixed siliciclastic-carbonate strata is an important type of tight oil resources.This type of tight oil reservoir is widely distributed across China and North America

and is believed to have great resource potential.Study on tight oil accumulation mechanism and main controlling factors for enrichment in mixed siliciclastic-carbonate strata are of great significance.The mixed strata of the Xiagou Formation (K

1

g)

Qingnan subsag

Qingxi Sag

Jiuquan Basin

are the case of study.We restored its burial history

thermal evolution history and hydrocarbon generation history via basin simulation techniques

investigated the formation of abnormal overpressure

pressure difference evolution between reservoir and source rock

and corresponding lower limit of pore throat for tight oil charging

and discussed the controls on hydrocarbon charging

seepage and enrichment in the mixed strata.Results show that the timing of hydrocarbon generation varies from different members in the formation:K

1

g

0

source rocks generated hydrocarbons early at the end of Early Cretaceous due to the high heat flow value in the early stage

while source rocks of K

1

g

2

and K

1

g

3

started to generate hydrocarbons late in the Neogene

and K

1

g

1

source rocks generated hydrocarbons in two periods(one early and one late period).Overpressure was believed to be attributed to hydrocarbon generation.Ever since 20 Ma

the remaining pressure on the boundary of source and reservoir rocks increased from 2 MPa to 32 MPa.The lower pore throat limit for oil charging on the boundary of source and reservoir rocks is 6 nm.Tight oil charging in the mixed strata of the Xiagou Formation and its short-distance migration accord with the invasion-percolation way

resulting in the pervasive accumulation of tight oil within and nearby the source rocks.The source-reservoir configuration and their internal architectures are the main controls on tight oil enrichment:the self-sourcing reservoirs and "sandwich-type"reservoirs are more oil-enriched than the lower-source upper-reservoir ones and thinly interlayered ones.